By using SEER-Medicare data, NCI would like to conduct a case-control analyses of screening endoscopy and colorectal cancer incidence and death that implement the Ko algorithm in order to define screening exposure. This will complement other ongoing work at NCI which uses an alternative approach to obtain an unbiased estimate of screening efficacy. The two alternative approaches can be tested side-by-side, using a single analytic dataset, to increase efficiency. Ultimately, this work would both provide additional information on whether colonoscopy is effective in preventing morbidity and mortality from right-sided colorectal cancer; it also may have additional applications outside of colorectal cancer screening due to its methodologically innovative method to assign exam indication.